The present invention relates generally to safety switch mechanisms, and in particular to a switch which is actuated by the insertion and removal of a probe.
Certain electronic test and measurement instruments, such as digital multimeters, are designed to facilitate a variety of measurement capabilities, including so-called floating measurements, e.g., measurements made with references to some potential other than earth ground. To this end, four input terminals are typically provided--high, low, guard, and chassis ground--which are electrically separated by predetermined impedances internal to the instrument. The actual measurement is made between the high and low inputs. A disconnectable bus bar is usually provided between the low and chassis ground terminals for DC-to-ground measurements and is disconnected for floating measurements. The guard terminal is electrically connected to a guard shield which is located physically adjacent the internal circuits to thereby establish a largely capacitive impedance between the low and guard inputs. Guarding is a passive technique to reduce common-mode noise between the high and low input terminals and chassis ground by shunting such noise-to-ground phenomena away from the input terminals. By rejecting common-mode noise in this fashion, higher-accuracy measurements may be made. The guard terminal may therefore be connected to an external guard voltage source, which may include a reference voltage within the circuit being measured and may even be the low input at the measurement source, to externally drive the guard shield.
Often it is desirable to connect the low input to the guard input within the instrument to short out the impedance therebetween and thus elevate the guard shield to the potential applied to the low terminal, particularly when no external guard voltage is applied. This may be achieved by the simple expedient of placing a switch between the low input and the guard input; however, other problems arise in that a shock hazard may be created at the unused guard terminal and that an externally-applied guard voltage may be shorted to a different potential applied via the switch to the guard shield.